ci: Fix meson-install usage check

The script has four mandatory arguments, and also accepts optional
build options that are passed on to meson. Checking for the number
of arguments *before* filtering out the optional ones means that

`./install-meson-project.sh -Done=1 -Dtwo=2 -Dthree=3 -Dfour=4`

is considered valid, even though not a single required argument
is passed.

Fix this by filtering out the arguments before doing the usage
check. As it is a nice touch to have usage information at the
top of the script, move the message into a usage() function at
the top.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2712>